Improve method to get list of active conductors

To facilitate the consistent hash ring, we must know not only what
conductors are active, but also which driver(s) each active conductor
supports. This patch renames list_active_conductor_drivers to
get_active_driver_dict and changes the return value to be a dict,
mapping drivers to the set of hosts which support them.

blueprint instance-mapping-by-consistent-hash

Change-Id: Ie327aa7d9207f6f5da1380631dbbc431dfd7e218

ironic/db/sqlalchemy/api.py

@@ -17,10 +17,10 @@
 
 """SQLAlchemy storage backend."""
 
+import collections
 import datetime
 
 from oslo.config import cfg
-
 from sqlalchemy.orm.exc import NoResultFound
 
 from ironic.common import exception
@@ -39,6 +39,9 @@ CONF = cfg.CONF
 CONF.import_opt('connection',
                 'ironic.openstack.common.db.sqlalchemy.session',
                 group='database')
+CONF.import_opt('heartbeat_timeout',
+                'ironic.conductor.manager',
+                group='conductor')
 
 LOG = log.getLogger(__name__)
 
@@ -544,15 +547,18 @@ class Connection(api.Connection):
             if count == 0:
                 raise exception.ConductorNotFound(conductor=hostname)
 
-    def list_active_conductor_drivers(self, interval):
-        # TODO(deva): add configurable default 'interval', somewhere higher
-        #             up the code. This isn't a db-specific option.
+    def get_active_driver_dict(self, interval=None):
+        if interval is None:
+            interval = CONF.conductor.heartbeat_timeout
+
         limit = timeutils.utcnow() - datetime.timedelta(seconds=interval)
         result = model_query(models.Conductor).\
                     filter(models.Conductor.updated_at >= limit).\
                     all()
 
-        driver_set = set()
+        # build mapping of drivers to the set of hosts which support them
+        d2c = collections.defaultdict(set)
         for row in result:
-            driver_set.update(set(row['drivers']))
-        return list(driver_set)
+            for driver in row['drivers']:
+                d2c[driver].add(row['hostname'])
+        return d2c